isl_set_from_pw_aff: explicitly convert result to set
isl_set_from_pw_aff: explicitly convert result to set

In the current code base, an isl_set is internally defined to isl_map,
but this may change in the future.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,Meinersbur/isl,Meinersbur/isl,inducer/isl-mirror,inducer/isl-mirror,PollyLabs/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,PollyLabs/isl